1. Field of the Invention
The present invention relates to an image quality stabilization technology for an image forming apparatus.
2. Description of the Related Art
Electrophotographic or inkjet image-forming apparatuses have widely been used. These image forming apparatuses are required to provide images with a constant level of quality. As one of causes of image deterioration, density unevenness (hereinafter referred to as “banding”) in a sheet conveyance direction (sub-scanning direction) can be considered. Under such circumstances, for example, Japanese Patent Application Laid-Open No. 2007-108246 proposes a solution to the banding in the sub-scanning direction. In Japanese Patent Application Laid-Open No. 2007-108246, first, banding in a sub-scanning direction occurring with a cycle corresponding to an outer diameter of a photosensitive drum is measured in advance in relation to phases of the photosensitive drum, and the measurement results are stored in a memory section as a density pattern information table. Then, when forming an image, banding information corresponding to the phases of the photosensitive drum is read out from the table, and based on the information, banding occurring with the cycle corresponding to the outer diameter of the photosensitive drum is corrected.
According to Japanese Patent Application Laid-Open No. 2007-108246, even though the mechanical precision is lowered, banding can be suppressed by means of electric image correction, so that costs required for the apparatus can be reduced.
Where, e.g., the temperature inside an image forming apparatus increases, a shaft and/or a drive gear in an electric motor may deform, resulting in variation in the amplitude and/or phase of rotation unevenness of each of such shaft and/or drive gear. Here, “Rotation unevenness” refers to periodic rotation speed variation. In such case, the technique of correcting image data such as in Japanese Patent Application Laid-Open No. 2007-108246 mentioned above has a problem in that a difference occurs between predicted banding and actually-occurred banding, resulting in an adverse increase in banding. The problem will be described in details below.
FIGS. 16A and 16B are diagrams each illustrating a relationship between predicted banding and actually-occurred banding. For example, it is assumed that densities of respective lines of a print image are predicted as indicated by predicted banding 2101 in FIG. 16A. Based on the predicted densities, the densities are corrected so as to cancel the banding. For example, for a scanning line with a high density in the predicted banding, like a scanning line L241, the image data is corrected so as to decrease the density, and meanwhile, for a scanning line with a low density in the predicted banding like a scanning line L242, image data is corrected so as to increase the density. Consequently, where there is almost no different in phase between the predicted banding 2101 and actually-occurred banding 2102 before correction as illustrated in FIG. 16A, the banding is cancelled so as to provide banding 2103 after correction, enabling suppression of banding. However, as illustrated in FIG. 16B, where there is a difference in phase between predicted banding 2104 and actually-occurred banding 2105 before correction, correction such as mentioned above results in an adverse increase in banding relative to the banding before correction. This will be described taking scanning lines L243 and L244 as an example. For a scanning line with a high density in the predicted banding like the scanning line L243, the corresponding image data is corrected so as to decrease the density. However, since a phase discrepancy occurs between the predicted banding and the actually-occurred banding, the actual density of the scanning line L243 is lower than an average density, and thus, the density is further decreased by the banding correction. Similarly, for a scanning line with a low density in the predicted banding like in the scanning line L244, because the actual density is higher than an average density, then the density is further increased by the banding correction. As a result, banding is adversely increased like banding 2106 by banding correction.